The purpose of the proposed study is to investigate the behavioral and anatomical manifestations of recovery of function after neural damage using the cat spinal cord as a model. The behavioral tests have revealed that, after hindlimb deafferentation, there are at least two types of locomotor recovery: 1) General ability to generate the locomotor pattern 2) Ability to locomote accurately. The first returns almost immediately while the second requires 8-10 post-operative days. Removal of afferent fibers in the ventral roots by ganglionectomy does not effect recovery and pre-operative training appears irrelevant. Postural reflexes are important but they are mediated by ipsilateral descending systems. Elimination of the latter by hemisection permanently abolishes all recovery from chronic deafferentation, thus demonstrating that the presence of contralateral afferent and descending systems is not sufficient to mediate the recovery. Locomotor recovery thus appears to be mediated specifically by ipsilateral systems; the thoracic dorsal roots which mediate postural reflexes and the ipsilateral descending systems. Both of these increase their projection fields during the recovery period due to collateral sprouting. A thoracic root sprouts locally, into its own and immediately adjacent segments and into parts of the nucleus gracilis. The descending tracts sprout into segments below the level of deafferentation. The two systems do not appear to overlap in the areas into which they sprout. The sprouting is thus limited perhaps by competition for denervated synaptic space. The recovery is also limited in that only certain of the remaining systems are able to mediate it.